The present invention relates to an arrangement for measuring the wavelength-dependent non-linearity of an optical spectrum analyzer (OSA) and, more particularly, to an arrangement which utilizes the essentially linear skirt portion of the OSA""s resolution bandwidth filter to assess the linearity of the OSA itself.
Optical spectrum analyzers are used for analyzing the output light beams from lasers, light-emitting diodes and other light sources. Optical spectrum analyzers are particularly useful for analyzing light sources for optical telecommunications, where it is preferable to ensure that the optical carrier includes only a single, spectrally pure wavelength. In optical spectrum analyzers, the light intensity of a light beam is displayed as a function of wavelength over a predetermined wavelength range. Parameters of importance include wavelength range, wavelength and amplitude accuracy, sensitivity, resolution, measurement speed, polarization insensitivity and dynamic range.
Optical spectrum analyzers frequently use a diffraction grating for separating the light beam to be analyzed into its component wavelengths. The input light beam to be analyzed is collimated and is directed at the diffraction grating. The light beam is spatially dispersed by the grating, since different wavelengths are diffracted at different angles. A resolution bandwidth filter is then used to provide an output at only the desired wavelength(s).
The results obtained from the optical spectrum analyzer are themselves limited by the accuracy of the components with the optical spectrum analyzer. Indeed, the wavelength non-linearity of a swept tunable filter with the optical spectrum analyzer adds uncertainty to any measurements being made by the analyzer. Prior attempts at deducing the wavelength non-linearity of an optical spectrum analyzer included using the peak level of the filter response. Unfortunately, the resolution of such a technique is limited due to the xe2x80x9cflat topxe2x80x9d response of the resolution bandwidth filter profile.
Thus, a need remains for an improved technique for assessing the non-linearity of an optical spectrum analyzer.
The need remaining in the prior art is addressed by the present invention, which relates to an arrangement for measuring the wavelength-dependent non-linearity of an optical spectrum analyzer (OSA) and, more particularly, to an arrangement which utilizes the essentially linear skirt portion of the OSA""s resolution bandwidth filter to assess the linearity of the OSA itself.
In accordance with the present invention, a wavelength source is coupled to the monochromator input of the OSA and an optical power meter is coupled to the monochromator output of the OSA. The wavelength source is capable of producing a range of optical signals from a predetermined xe2x80x9cstartxe2x80x9d wavelength (for example, 1530 nm) to a predetermined xe2x80x9cstopxe2x80x9d wavelength (for example, 1570 nm). The insertion loss of the spectrum analyzer, over the wavelength range of interest, is first determined and stored in an array. The source wavelength is then measured over a span sufficient to measure the amplitudexc2x110 dB from the peak value. The diffraction grating is then stepped from a first one of these xe2x80x9cedgexe2x80x9d values and across the peak to determine the 3 dB point. The wavelength and amplitude are recorded at each step from the xe2x80x9cstartxe2x80x9d wavelength to the xe2x80x9cstopxe2x80x9d wavelength.
In order to ascertain the non-linearity of the optical spectrum analyzer, the insertion loss value is first subtracted from its associated amplitude measurement, the resultant corrected amplitude then be normalized against the prior recorded peak value. The skirt slope (in percent/pm) is then determined. The actual xe2x80x9cwavelength offsetxe2x80x9d can then be ascertained, as a function of wavelength by subtracting the peak amplitude value from the normalized value and dividing by the skirt slope.
Other and further aspects of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.